ozone: Add README files for subdirectories

Not everything is in the right place yet, but this is the general idea.

NOTRY=true

Review URL: https://codereview.chromium.org/342983005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278753 0039d316-1c4b-4281-b951-d872f2087c98

ui/ozone/common/README

+This directory contains code that is used by multiple platforms, but is not part
+of the public platform API. This code should not be used outside of ui/ozone.

ui/ozone/platform/README

+This directory contains implementations of platforms. Each platform implements
+the interfaces from ui/ozone/public for the rest of chromium.

ui/ozone/public/README

+This directory contains the platform API exposed to higher level components such
+as content. These interfaces can be used outside of ui/ozone.